It is important to match up the type of cloth you are using with the correct heat transfer product. For instance, the slick bumper material from Andy Mark is made of nylon, which is a difficult product to stick most heat transfer vinyl to.
This chart from Siser, manufacturer of Easy Weed heat transfer vinyl, shows the different types of product they make and what fabrics they work on. Only Easy Weed Extra, VideoFlex Glitter, and Glitter adhere to nylon, and it specifies "lightly siliconed vinyl," which I don't think describes the slick bumper material.
https://www.specialty-graphics.com/m...vinyl-tech.pdf
If you think it's the type of fabric you have, you might try swapping out your fabric for cotton duck cloth, widely available in red and blue at JoAnn Fabric. Use a coupon and it won't be too costly.
When vinyl type and fabric type are a good match, there are three other things that can affect the end product: proper temperature, dwell time, and pressure. Basic Easy Weed for cotton works at 305 degrees, requires medium pressure, and 10-15 seconds. We've had to go up to 320 degrees, heavy pressure (using a heat press), and 30-40 seconds for some heavy cotton fabrics.